Rate Coefficients and Branching Ratio for Multi-Channel Hydrogen Abstractions from CH3OH by F

摘要

The hydrogen abstraction reaction F+CH3OH has two possible reaction pathways:HF+CH3O and HF+CH2OH.Despite the absence of intrinsic barriers for both channels,the former has a branching ratio comparable to the latter,which is far from the statistical limit of 0.25 (one out of four available H atoms).Furthermore,the measured branching ratio of the two abstraction channels spans a large range and is not quantitatively reproduced by previous theoretical predictions based on the transition-state theory with the stationary point information calculated at the levels of Mφller-Plesset perturbation theory and G2.This work reports a theoretical investigation on the kinetics and the associated branching ratio of the two competing channels of the title reaction using a quasi-classical trajectory approach on an accurate full-dimensional potential energy surface (PES) fitted by the permutation invariant polynomial-neural network approach to ca.1.21× 105 poi.nts calculated at the explicitly correlated (F12a) version of coupled cluster singles doubles and perturbative triples (CCSD(T)) level with the aug-cc-pVDZ basis set.The calculated room temperature rate coefficient and branching ratio of the HF+CH3O channel are in good agreement with the available experimental data.Furthermore,our theory predicts that rate coefficients have a slightly negative temperature dependence,consistent with barrierless nature of the reaction.